Cancer cells possess multiple mechanisms to suppress the cytotoxic effects of DNA-damaging chemotherapeutic drugs that limit their effectiveness with concomitant increases in the mutational burden to all tissues. This added genomic instability is expected to select for chemoresistant cancer cells and foster secondary tumor formation. Since DNA alkylating agents comprise a significant portion of the available chemotherapeutic drugs, knowledge of the spectrum of biologically relevant DNA lesions that are created during therapy and the biological processing of these adducts is critical for the design of more effective treatments. For alkylating agents such as temozolomide, thioTEPA, and nitrogen mustards, the most abundant lesion is formed at N7-guanine, with less abundant, but biologically active lesions forming elsewhere. A similar spectrum of DNA lesions is also formed from exposures to environmental toxicants that have known cancer etiologies. The imidazole portion of N7-alkylated guanines can undergo base-induced ring-opening, yielding stable alkyl-formamidopyrimidine (N5-substituted-Fapy) lesions. The central hypothesis of this program project is that the role of Fapy-dG lesions in modulating genotoxic responses has been overlooked and that the Fapy-dG lesions contribute substantially to the biology associated with the DNA damaging agents. A major reason why Fapy lesions have been under-studied has been an inability to prepare DNAs containing them for biological, biochemical, and structural studies. Insights gained from our multidisciplinary, interdependent approaches will yield fundamental and applied understanding of 1) the identities of stable alkyl-Fapy-dG adducts and their detection in cellular DNA, 2) routes of chemical synthesis for the production and characterization of adduct-containing DNAs, 3) structural understanding of how these modified DNAs not only alter the structure of DNA, but also interface with DNA repair and replication enzymes, and 4) the biological processing of these DNAs by various repair systems to limit cytotoxicity and mutagenesis, or replication bypass to promote damage tolerance and survival, while increasing mutagenesis.